1. Field of the Invention
This invention relates generally to antennas that exhibit wide bandwidth and wide beamwidth, and more specifically relates to wideband planar antennas. Even more particularly, the present invention relates to multi-octave bandwidth spiral antennas, log-periodic antennas and sinuous antennas.
2. Description of the Prior Art
The multi-octave bandwidth spiral antenna is a preferred antenna-type for Electronic Warfare Support Measures (ESM) and ELectronic INTelligence (ELINT) radar systems. The reasons for choosing a spiral antenna over others are that its wide bandwidth offers a high probability of intercept, and its wide beamwidth is well matched to either the field-of-view requirements of a wide-angle system or to the included angle of a reflector in a narrow field-of-view system. Nevertheless, the spiral antenna does have a significant fault; its efficiency is less than fifty percent since it invariably depends on an absorber-filled back cavity for unidirectionality.
The conventional, planar, two-arm, spiral antenna comprises two planar conductors that are wound in a planar, bifilar fashion from a central termination. At the center of the spiral antenna, a balanced transmission line is connected to the arms of the antenna and projects at right angles to the plane of the spiral. The conductive arms of the spiral antenna are wound outwardly in the form of either an Archimedes or equiangular spiral. Stated differently, the radial position of either winding is linearly proportional to the winding angle, or its logarithm in the case of the equiangular spiral antenna.
The spiral antenna is typically used as a receiving antenna. However, the operation of the spiral antenna is more easily explained by considering the spiral antenna as a transmitting antenna. A balanced excitation applied to the central transmission line induces equal, but oppositely-phased, currents in the two conductive arms near the center of the spiral. The two currents independently progress outwardly following the paths of their respective conductive arms. Eventually, the currents progress to the section of the spiral that is approximately one free-space wavelength in circumference. In this section, the differential phase shift has progressed to 180 degrees so that the adjacent conductor currents which started in opposition are now fully in phase. Furthermore, the currents in diametrically opposing arc sections of the spiral antenna are now co-directed because of a phase reversal, which enables strong, efficient broadside radiation from these currents.
The region of efficient radiation of the spiral antenna scales in physical diameter with operating wavelength. Thus, a spiral antenna comprising many windings (i.e., greater physical diameter) has a large bandwidth. The spiral antenna radiates efficiently in both forward and backward directions normal to its plane. If only forward coverage is desired, then the backward radiation is wasted, resulting in a 3 dB decrease in efficiency, and a directive gain of only about 2 dBi.
In addition to the loss in efficiency, portions of the backward radiation can also be reflected or scattered forward by structures behind the spiral antenna. This forward-scattered radiation interacts with the directly-forward radiation to cause scalloping of the forward pattern. Thus, in those cases where the spiral antenna must be located in front of other structures, the spiral winding is typically backed by a microwave absorber within a metallic cavity. The microwave absorber and the metallic cavity increase shielding and provide environmental protection.
Previous attempts to render the spiral unidirectional without this 3 dB loss resulted in limiting its bandwidth. For example, by removing the absorber and retaining the cavity (or including a rear ground plane), the gain is increased to approximately 5 dB. However, this reduces the bandwidth to less than an octave, even if the spiral is optimally spaced from the back wall of the cavity. In one method to achieve wider bandwidth without the absorber lining, the spiral-to-backwall spacing is increased with spiral radius so that the spacing is optimal in the radiating region (i.e., where the windings are one wavelength in circumference), regardless of the frequency. In other words, the back wall is conically concave in shape. This method is not fully acceptable because a substantial portion of the backward radiated signal propagates radially outward from the sloping cavity backwall, until it is reflected by the cavity sidewalls.
A microstrip version of the spiral antenna was also attempted. This structure is distinguished by its use of material with a high dielectric constant and low loss to fill the space between the spiral antenna and the cavity backwall. This structure also fails to achieve a greater-than-octave bandwidth since most of the radiation is directed into the substrate rather than into the air, and much of the substrate signal is trapped in the radial propagation of a surface wave.